Improve the documentation structure of the Dhall module

[sjakobi]

The Dhall module has become quite large. It would be nice if it would be easier to navigate.

[sjakobi]

A detailed pass over the existing haddocks would also help!

[Gabriella439]

Does #2193 fix this?

[sjakobi]

Does #2193 fix this?

I think it's a great improvement. I can open a new issue if I find anything left that should be improved.